Inherently conductive polymers (“ICPs” or “conductive polymers”) are polymers that conduct electricity. Examples of conductive polymers include electroactive polymers that exhibit a change in a physical property when stimulated by an electric current. Electrochromic polymers are an example of electroactive polymers that are commonly used in windows, mirrors and displays. Conductive polymers have been used in antistatic materials, displays, batteries, photovoltaic devices, printing electronic circuits, organic light-emitting diodes (OLEDs), organic transistors (OFETs), actuators, electrochromic applications such as windows, mirrors and displays, supercapacitors, biosensors, flexible transparent displays, electromagnetic shielding, and as a replacement for the transparent conductor indium tin oxide.
Conductive polymer films can be prepared by dissolving organic polymers in a solvent, casting or coating the resulting solution onto a substrate, and removing the solvent to form a substrate having the conductive polymer on the surface of the substrate. An example of a conductive polymer is polyaniline which can be formed either by the electrochemical or chemical oxidation of aniline.
Among the different conductive polymers used in practical applications, poly(3,4-ethylenedioxythiophene) (PEDOT) is a relatively stable inherently conductive polymer. PEDOT can be formed by oxidatively polymerizing 3,4-ethylenedioxythiophene (EDOT) monomer by wet chemical polymerization, electrochemical polymerization, chemical vapour deposition or vapour phase polymerization. A common method for obtaining PEDOT films is to polymerize EDOT via wet chemical oxidation and then apply the polymer to a substrate using a suitable coating process, such as dip coating, spin coating, printing, spray coating, etc.
Electrochemical polymerization, can also be used to deposit PEDOT films on substrates. Thus, EDOT monomer can be coated onto a conductive substrate and the sample placed into a three electrode cell with an electrolyte. A periodic voltage can be cycled across the cell, with each cycle increasing the amount of PEDOT deposited on the substrate. After 20-40 cycles film growth is generally finished. Whilst the PEDOT films formed using this method are generally high in quality, the deposition technique only allows for PEDOT films to be formed on conductive substrates and the technique is not well suited to large scale and/or commercial applications.
PEDOT films have also been formed on substrates by vapour phase polymerization of EDOT. In this method, a substrate is coated with an oxidant and the oxidant coated substrate is placed in to a chamber containing EDOT monomer in the vapour phase. Under appropriate conditions the monomer condenses on the substrate and polymerizes, creating a highly conductive and homogenous PEDOT film.
We have previously developed a process for producing an electrochromic substrate by vapour phase polymerisation of PEDOT (see WO 2009/117761, the contents of which are incorporated herein in their entirety). However, we have found that there are processing difficulties associated with prior art methods for the vapour phase polymerisation of PEDOT. In particular, the conductivity of the PEDOT polymers formed was found to be highly dependent on the deposition conditions. For example, the methods disclosed in WO 2009/117761 require the addition of water in vapour form during the polymerisation process.
There is a need for processes for producing conductive polymer films that have improved conductivity over prior art conductive polymer films. Alternatively, or in addition, there is a need for processes for producing conductive polymer films that overcome one or more of the problems associated with prior art processes.